Chemistry Reference
In-Depth Information
product not only formed through a nitrene intermediate but there was also a
concerted pathway that bypassed the nitrene. Along the same lines, Harger reported
a photolysis study of
t
-butyl(mesityl)phosphinic azide, which similarly concluded
that both concerted and stepwise mechanisms are possible.
167
Which excited state of
the phosphyl azides is involved in nitrene formation? Is it the same excited state,
which follows the concerted rearrangement mechanism? Several such mechanistic
questions remain unanswered.
CBS-QB3 calculations suggested that the free energy of activation for the
concerted rearrangement is 45.4 and 47.0 kcal/mol for dimethylphosphinyl and
dimethylphosphoryl azides, respectively.
24
The authors could not locate analogous
transition states for the case of dimethylphosphinoyl azide at the CBS-QB3 level;
however, an estimated barrier of 80 kcal/mol was predicted at the B3LYP/6-311
G
(
d
,
p
)//B3LYP/6-31G(
d
) level of theory. Interestingly, the transition state for the
concerted process on the triplet surface could not be obtained for dimethylphos-
phinyl and dimethylphosphinoyl azide, while dimethylphosphoryl azide has a barrier
of 75.8 kcal/mol at the CBS-QB3 level. It was predicted that both phosphinoyl and
phosphoryl azides should follow primarily nitrene chemistry, while phosphinyl
azides will provide a mixture of nitrene chemistry and Curtius rearrangement.
24
Although all of these calculations are quite extensive, there has been limited
computational work on the rearrangements for the excited state potential energy
surfaces; thus, there is little insight into the photochemical behavior of phosphyl
azides.
Houser et al.
168
reported nanosecond laser flash photolysis measurements on
diphenylphosphoryl azide in alcoholic solvents, for which they observed the triplet
phosphorylnitrene. Subsequently, Gohar and Platz
23
reported laser flash photolysis
studies of the same phosphoryl azide in different solvents, and suggested that the
lifetime of the triplet nitrene is highly solvent-dependent. They estimated the lifetime
of the singlet diphenylphosphorylnitrene to be about 1 ns using a Stern-Volmer
analysis. However, all of these studies have investigated the nitrene products, while
the ultrafast measurements have not yet been reported for these phosphyl azides so as
to characterize the excited state of the azide precursors. Our TD-B3LYP/TZVP
calculation on dimethylphosphoryl azide located the S
1
and S
2
states at about 242
and 212 nm above the S
0
ground state (S. Vyas and C. M. Hadad, unpublished
results). Difference density analysis of these excited states suggest that the S
1
state is
a(
รพ
p
) state localized on the azide unit which eventually may lead to the formation
of nitrene while the S
2
state is a mixture of two orbital-to-orbital transitions
(Fig. 2.15).
Recently, we reported ultrafast time-resolved absorption measurements on diphe-
nylphosphoryl azide along with
ab initio
calculations.
169
The computational work
suggested that there are two possible mechanisms for singlet phosphorylnitrene
generation. The TD-B3LYP/TZVP calculations suggested the precursor azide is
excited to the azide dissociative state that then leads to the singlet nitrene; however,
the RI-CC2/TZVP level of theory predicted that initial excitation to the phenyl
(
p
,
p
) state occurs first and then after energy transfer, the azide dissociative state
is generated,
p
,
thereby leading to the singlet phosphorylnitrene. Femtosecond
